1. Field of the Invention
This invention relates to the control of a beam of energy. More particularly this invention relates to the controlled motion of a focal point of a beam of light to selected positions in a multilayered optical information medium.
2. Description of the Related Art
Digital optical media such as optical discs and optical tapes are now commonly used for mass storage of information, for example compressed MPEG encoded audio and video signals. The information storage capacity of the discs and tapes can be enhanced by layering a plurality of information containing layers on a substrate. To read a multilayered optical medium, a focal point of light is selectively positioned on the layers, and is shifted from layer to layer in accordance with the format in which the media has been written. Shifting of the focal point is generally accomplished by arrangements requiring mechanical movement of the optics relative to the medium. This requires a large separation between layers to assure differentiation of the individual layers by an optomechanical link and its associated servo circuitry.
Focus acquisition is conventionally accomplished by various servo arrangements initially operating in an open loop mode. The feedback loop of the focus servo includes a switch that is initially open, during which time the focus servo is driven by an oscillating waveform, such as a sawtooth waveform, causing an objective lens to be displaced toward and away from the optical medium, and a beam of light passing through the objective lens goes in and out of focus on the medium. At some point, when the beam is near a desired focal position, the sawtooth waveform is removed and the switch closed, thereby closing the feedback loop. Typical is the disclosure of Wachi, U.S. Pat. No. 5,379,282, which proposes the use of detectors that detect maxima and minima of return light from the optical medium, and maxima and minima of a focus error signal. These maximum and minimum signals are processed by a servo, which drives a focus actuator. A focus servo operation then executes to lock in the light focus at a desired level.
In Millar et al., U.S. Pat. No. 4,607,157, it is proposed to intentionally defocus the light spot on an optical disk after focus acquisition has been achieved and while the servo is operating in closed loop mode. The resulting variation in the read-back signal is utilized by a synchronous detection circuit to extract magnitude and polarity information of the focus offset. This is fed back to the focus servo signal to null out the defocussing effect, and restore focus lock.
A conventional focus control circuit 10 is illustrated in FIG. 1, wherein a astigmatic optical pickup 12 comprises a matrix of four photoelectric transducers 12a-12d, arranged to detect a light beam that returns from an optical information medium through an objective lens (not shown). It will be understood herein that the objective lens is a component of a known optomechanical link 28 between the focus control circuit 10 and the optical pickup 12. Signals from paired, diagonally opposed transducers (pair 12a, 12d and pair 12b, 12c) are combined on lines 13a, 13b respectively, and amplified respectively by operational amplifiers 14a, 14b. The paired signals on lines 13a, 13b independently vary as the focal point of the objective lens transits the information layer of an optical medium, and these signals are responsive to the focus offset of the objective lens from the information layer. The outputs of the operational amplifiers 14a, 14b drive a differential amplifier 16, which outputs a focus error signal on line 32. The focus error signal on line 32 is representative of the difference between the signals on lines 13a, 13b. In closed loop operation, the focus error signal on line 32 is coupled to conventional phase and gain compensating circuitry, referred to herein as servo circuitry 18. Servo circuitry is disclosed, for example, in Ceshkovsky et al., U.S. Pat. No. 4,332,022. The focus error signal on line 32 is an input to the servo circuitry 18, and causes a modification in its behavior in accordance with the loop design. The output of servo circuitry 18 is summed with the output of focus acquire control circuitry 20 in summing circuit 22. The output of the summing circuit 22 is amplified in a drive amplifier 24, and coupled to a focus actuator, represented as actuator coil 26. The optomechanical link 28 between the actuator coil 26 and the optical pickup 12 is indicated by a dashed line.
Initially switch 30 is opened by a control means (not shown), so that the focus error signal on line 32 is disengaged from the servo circuitry 18, but remains coupled to the focus acquire control circuitry 20 via line 34. In this circumstance, the focus actuator coil 26 is driven by an oscillating waveform added on summing junction 22, and the optomechanical link 28 moves an objective lens (not shown) generally toward and away from the surface of the optical medium. The output of the optical pickup 20 varies as the focal point of the objective lens approaches an information layer of the optical medium. When the lens is approximately in focus on the information layer, the switch 30 is closed, and the servo circuitry 18 begins closed loop operation.
When a light beam is perfectly focused on an information layer of an optical medium, the light intensity on the paired photodetector elements 12a, 12d and 12b,12c of the optical pickup 12 is equal. The signals on lines 13a, 13b, and the signals developed by the operational amplifiers 14a, 14b are also equal, and the output of the differential amplifier 16 is nominally zero. As the focal point of the objective lens drifts away from the information layer, the intensity of light measured by the pairs of photodetector elements varies, so that the signals on lines 13a and 13b become unequal, and the differential amplifier 16 generates a focus error signal on line 32 that has a voltage level either greater than zero, or less than zero, depending on the direction the focal point of the objective lens has moved from the information layer.
Fundamental principles underlying the invention are also disclosed in copending application Ser. No. 08/474,424, of common assignee herewith.
A typical waveform plot of a focus error signal according to the circuit of FIG. 1 is shown as waveform 50 in FIG. 3, wherein F1 and F2 indicate the positions of two information bearing layers on a multilayered optical medium. When the focal point of the objective lens is remote from the information layer of the optical medium, for example at the left side of focus error waveform 50, the focus error signal has a baseline value. As the focal point of the objective lens approaches the first information layer F1, in a direction indicated by arrow A, the differential amplifier 16 begins to develop a positive signal, which is approximately sinusoidal, and which returns to the baseline value when the objective lens focal point actually crosses the first information layer F1 at point 52. As the objective lens continues traveling beyond the first layer, the differential amplifier 16 produces a signal which is less than the baseline value. When the objective lens is sufficiently remote from the first information layer F1, the focus error signal again returns to baseline. The above sequence is repeated as the focal point of the objective lens transits a second information layer F2, with a zero crossing occurring at point 54.
With the above noted approaches it is necessary to return to an open loop mode of operation when it is desired to shift focus from a first information layer to a second information layer, and to reclose the loop in order to lock focus on the second layer. Otherwise the servo loop would initially resist movement to the second information layer, and eventually be overcome, after which the focus would move in an uncontrolled manner.